Minor updates to inline comments

Add missing docstring on InstallableCommand. Also, some of these were wrapped
when they're right next to a line longer than the unwrapped line, so we can just
unwrap them to save vertical space.

src/libstore/local-store.cc

@@ -937,8 +937,7 @@ void LocalStore::registerValidPaths(const ValidPathInfos & infos)
            not be valid yet. */
         for (auto & i : infos)
             if (isDerivation(i.path)) {
-                // FIXME: inefficient; we already loaded the
-                // derivation in addValidPath().
+                // FIXME: inefficient; we already loaded the derivation in addValidPath().
                 Derivation drv = readDerivation(realStoreDir + "/" + baseNameOf(i.path));
                 checkDerivationOutputs(i.path, drv);
             }